1. Field of the Invention
This invention relates to surface hardened zirconium-containing titanium medical implants, and in particular such hardened implants where the surface and near surface region is comprised of a mixed-oxide surface layer and an underlying near-surface-oxygen-rich solution layer. A zirconium-rich interface develops, in some instances, between the mixed oxide layer and the oxygen-rich solution. In particular, these surface hardened zirconium-containing titanium implants may be produced by an elevated temperature process which permits the diffusion of oxygen into the near surface of the implants. This invention is especially useful for medical implants but is also useful for other applications in which improvement in wear resistance of titanium alloys is beneficial.
2. Description of the Related Art
Titanium alloys are used extensively in medical implants such as hip joint prostheses, knee joint prostheses, spinal implants, bone plate systems, intramedullary nails, dental implants, cardiovascular implants, ear-nose-and-throat implants, etc., due to their high strength, low modulus, excellent biocompatibility and corrosion resistance. However, a major disadvantage of titanium alloys is their susceptibility to wear and galling.
Minimizing wear debris generated by orthopedic devices, such as hip joint and knee joint prostheses, is an issue of concern in orthopedics. Wear debris generated by orthopedic devices has been associated with a phenomenon called "osteolysis," a term used to describe death of bone cells. This can lead to premature loosening of an orthopedic implant from surrounding bone and subsequent failure of the device.
Titanium alloys are also susceptible to a phenomenon called "galling," essentially the sticking together of mating titanium parts which move against each other leading to high friction and wear.
Numerous methods have been proposed for increasing the surface hardness and reducing the wear and galling of titanium alloys. Ceramic coatings, such as titanium nitride, have been deposited on these alloys by processes such as physical vapor deposition and chemical vapor deposition (see, for example, Hintermann, U.S. Pat. No. 4,687,487). However, these ceramic coatings are much harder and stiffer than the base alloy substrate so that there is an abrupt mismatch in the stiffness of the coating and the substrate at the interface between the two. The elastic modulus (stiffness) of a titanium nitride coating is typically about 400 GPa while that of most titanium alloys is about 65 to 130 GPa. This modulus mismatch leads to undesirable stresses at the interface, especially when these components are bent or deformed in any manner, and increases the potential for the coating separating from the substrate by a delamination or spalling mechanism.
Attempts have been made to harden titanium alloys by nitrogen or oxygen ion implantation. In these processes, the titanium alloy substrate is bombarded with nitrogen or oxygen ions using a high voltage apparatus which forces the ions to penetrate the substrate. However, these processes affect the surface to a depth of only about 0.1 micron and peak hardness is not at the surface but slightly below the surface. Hence, the hardened surface tends to wear through relatively quickly.
Titanium alloys have also been hardened by processes such as gas nitriding and salt bath nitriding. These processes also produce a titanium nitride surface on these alloys by penetration of nitrogen into the metal substrate. However, as mentioned above, titanium nitride has a much higher stiffness than the titanium alloy base material, thus being potentially susceptible to detachment from the substrate by delamination or spalling.
There have been a few attempts at oxygen diffusion hardening of titanium alloys, such as for instance disclosed by Streicher, et al., in two conferences (CIMTEC, 1990, Italy, and European Society of Biomechanics, July 8-11, Denmark). However, the alloy used by Streicher, et al., is Ti--6Al--7Nb, which when oxidized would be expected to produce titanium oxide (TiO) or titanium dioxide (TiO.sub.2), both of which have very low shear strength and would be susceptible to detachment.
British Patent No. 1351062 discloses a process for surface hardening a titanium article by heating it in an atmosphere of air, nitrogen, hydrogen or oxygen. However, if heated in air, the surface would be expected to consist of titanium oxide (TiO), titanium dioxide (TiO.sub.2), or titanium nitride with the associated disadvantages described above. If heated in nitrogen, the surface produced would consist of titanium nitride with the associated stiffness mismatch disadvantage described above. If heated in hydrogen, the compound produced would be titanium hydride which is known to severely embrittle and be detrimental to the fatigue strength of titanium alloys. If heated in oxygen, the surface would be expected to consist of titanium oxide (TiO) or titanium dioxide (TiO.sub.2) with the associated low shear strength described above.
Clearly, the development of an effective means for increasing the surface hardness and wear resistance of titanium alloys would be extremely beneficial. In the case of medical implants, abrasive wear may be minimized or eliminated by increasing the hardness of the surface of a titanium alloy. A highly wear resistant titanium implant will not only produce less wear debris which will increase the expected service life of the implant, but will result in reduced levels of metal ion release into the recipient's body tissue. Further, a longer lasting implant may reduce the need for later surgery to replace the implant. Such a process would also be extremely beneficial in reducing wear and galling in non-medical applications of titanium alloys.